This invention concerns control of the volume of air flowing through a duct. More particularly it concerns the combination of various features in a control, whereby a small actuator consuming low power may be used.
Prior systems have employed butterfly dampers, pivoted near their midpoints to control air flow through ducts. Some form of power must be supplied to move such a damper toward both full open and closed positions. It is common to employ a controlled actuator to move the damper toward full open position in opposition to a means biasing the damper toward closed position. One example of such a biasing means is a return spring. In order to exert equal forces in opposite directions, return springs are designed to exert half the force produced by the actuator. This wastes half of the power of the actuator in compressing the return spring and thus makes a larger, more powerful actuator necessary.
Roger P. Engelke and Marvin H. Zille, in U.S. Pat. No. 3,809,314, describe a damper control employing a pneumatic bellows to move the damper toward full open position and, although they did not say so, relying upon the weight of the actuator, its connecting linkages and sometimes auxiliary weights to bias the damper toward closed position. They did not rely upon aerodynamic forces for biasing. In other respects the control disclosed by them is similar to that described and claimed herein.
Albert W. Schach in U.S. Pat. No. 3,361,157, reissued as U.S. Reissue Pat. No. 26,690, described a butterfly damper pivoted at its center and always biased (he believed) toward closed position by unbalanced aerodynamic forces resulting from air flow past the damper. The damper was operated, in supposed opposition to the bias, by a small pneumatic bellows with no return spring. Operation was unstable with a tendency for the damper to oscillate. The reasons for these oscillations were not known. A more conventional damper operator, with large bellows and not relying upon a bias produced by aerodynamic forces, was substituted. Oscillation of the damper may produce widespread undesirable consequences. Not only does it produce undesirable fluctuating air flow into the space being supplied, but these fluctuations are detected by the flow sensor controlling the bellows, causing it to move in response to the fluctuations. This may reinforce the original oscillations, producing sustained oscillation and thus aggravating the undesirable condition. Usually a plurality of air flow controllers control air flow through respective ducts supplied with air from a common plenum. When the damper in one controller begins to oscillate, it produces upstream pressure variations, which are transmitted to the other dampers. These other dampers may then begin to oscillate, producing the undesirable fluctuating air flow into the spaces supplied by the respective ducts. Cases have been known in which the entire air distribution system began to oscillate in synchronism. More conventional volume air flow controllers employ actuators and return means so powerful that they swamp out the effect of unbalanced aerodynamic forces on the damper. They avoid sustained oscillations produced by the unbalanced aerodynamic forces, but require relatively large and powerful operating means.